As mentioned above, the channel estimation for the RIS related channels \(\bf {G}\)  and \({\bf {h}}_r\)  is challenging. A straightforward solution is to estimate the cascaded channel \(\bf {H}\)  in (REF ) based on the ON/OFF protocol proposed in [1]}. The key idea is to divide the entire cascaded channel estimation process into \(N\)  stages, where each stage only estimates one column vector of \({\bf {H}}\in {\mathbb {C}}^{M\times N}\)  associated with one RIS element. Specifically, the cascaded channel \({\bf {H}}\in {\mathbb {C}}^{M\times N}\)  can be represented by \(N\)  columns as
\({\bf {H}}=\left[\mathbf {h}_{1 }, \cdots ,\mathbf {h}_{n},\cdots , \mathbf {h}_{N}\right],\) 
